Britain is about to add up to a million and a half people to its voter rolls. Some lean left, some lean right and others don’t care about politics at all. But all of them are 16 or 17 years old. Liberals are thrilled with the plan, which lawmakers announced yesterday; conservatives are outraged.

In planning to lower the national voting age before the next general election, Britain joins a small but growing club of nations willing to test the boundaries of electoral inclusion. Brazil, an early adopter, cut its voting age to 16 in 1988. Austria, Argentina and Malta followed suit in the 2000s. Most recently, Germany and Belgium decided to let 16- and 17-year-olds vote in some elections but not others.

Beneath these changes lies a fundamental question: At what age does civic responsibility begin? Today, I explain the debate and look at how it is reshaping some democracies, including America’s.

Who’s ready?

Legal codes are filled with age minimums. In the United States, you must be 18 to serve in the military, 21 to drink alcohol and 35 to run for president. Sometimes, these are built on easy-to-understand ideas. Alcohol, for instance, is meant only for people who are likely to drink responsibly.

The right to vote follows from the notion that governments draw their legitimacy from the consent of the governed. When can someone give that consent? Experts offer different answers to the question.